Just recently I started writing a little web application (www.instalog.me). I started doing it very classical in PHP, because it's just so simple. It worked perfectly fine and it was actually fairly performant. It's only running on a cheap shared hosting. So I was thinking: "What will happen if that service would become popular?". The answer: "Caching and moving to a new bigger server."
Is this realistic?
After all that kind of application will provide a better user experience, they are faster and they are cheaper to run, but it also takes some effort to develop.